1. Field of the Invention
The present invention relates to an exchange coupling film which has an antiferromagnetic layer and a ferromagnetic layer, wherein the direction of magnetization of the ferromagnetic layer is fixed in a predetermined direction by the effect of an exchange magnetic field which is generated at the interface between the antiferromagnetic layer and the ferromagnetic layer. More particularly, the present invention relates to an exchange coupling film which is improved to exhibit a large ratio of resistance variation and also to a magnetoresistive sensor, e.g., a spin valve thin-film device or an ARM device, incorporating such an exchange coupling film, as well as to a thin-film magnetic head which uses such a magnetoresistive sensor.
2. Description of the Related Art
A spin-valve-type thin-film device is a kind of GMR (Giant Magnetoresistive) device which makes use of a giant magnetoresistive effect, and is used for detecting recording magnetic fields from a recording medium such as a hard disk.
The spin-valve-type thin-film device, among various GMR devices, has advantageous features such as simplicity of the structure and high subtlety to vary its magnetic resistance even under a weak magnetic field.
The simplest form of the spin-valve-type thin-film device has an antiferromagnetic layer, a pinned magnetic layer, a non-magnetic intermediate layer, and a free magnetic layer. The antiferromagnetic layer and the pinned magnetic layer are formed in contact with each other, and the direction of the pinned magnetic layer is put into a single magnetic domain state and fixed by an exchange anisotropic magnetic field, which is produced at the interface between the antiferromagnetic layer and the pinned magnetic layer. The magnetization of the free magnetic layer is aligned in a direction which intersects the direction of magnetization of the pinned magnetic layer, by the effect of bias layers that are formed on both sides of the free magnetic layer.
Alloy films such as an Fexe2x80x94Mn (Iron-Manganese) alloy film, Nixe2x80x94Mn (Nickel-Manganese) alloy film and a Ptxe2x80x94Mn (Platinum-Manganese) alloy film are generally usable as the material of the antiferromagnetic layer, among which the Ptxe2x80x94Mn alloy film are attracting attention due to its advantages such as a high blocking temperature, superior corrosion resistance, and so forth. It has been recognized that, when a Ptxe2x80x94Mn alloy film is used as the material of a ferromagnetic layer, the film as deposited has a crystalline structure composed of face-centered cubic lattice in which atoms are positioned in an irregular manner.
In order that a large exchange coupling magnetic field is generated between a ferromagnetic layer and an antiferromagnetic layer after deposition, it is necessary that the crystalline structure of the antiferromagnetic layer be transformed from face-centered cubic lattice as disordered phase to a CuAuxe2x80x94I face-centered square lattice as an ordered phase. Such a transformation can be effected by a heat treatment.
It has been recognized also that a Ptxe2x80x94Mn alloy of bulk type is easily transformed into CuAuxe2x80x94I face-centered square lattice to maximize the antiferromagnetic properties when the ratio of content between Pt and Mn is 50:50 in terms of atomic percent (at %). With this knowledge, the present inventors have made spin valve thin-film device having an antiferromagnetic layer composed of a Ptxe2x80x94Mn alloy, the content ratio between Pt and Mn being set substantially at 50:50, and measured the strength of the exchange magnetic field generated between the antiferromagnetic layer and a ferromagnetic layer. As a result, the inventors found that the strength of the exchange coupling magnetic field is still unsatisfactory, despite the use of the composition ratio between Pt and Mn which is ideal for a bulk phase recrystalization. This is attributable to the fact that the transformation from a disordered lattice to an ordered lattice is still insufficient despite of the heat treatment.
An object of the present invention is to provide an exchange coupling film capable of generating a large exchange coupling magnetic field when an element X, wherein X is a platinum-group element and Mn comprise an antiferromagnetic layer. A further object is to provide magnetoresistive sensor using such an exchange coupling film and also a thin-film magnetic head incorporating such a magnetoresistive sensor, thereby overcoming the above-described problems of the prior art.
To this end, according to the present invention, there is provided an exchange coupling film comprising: an antiferromagnetic layer; and a ferromagnetic layer in contact with the antiferromagnetic layer such that an exchange coupling magnetic field is produced at the interface between the antiferromagnetic layer and the ferromagnetic layer to fix the magnetization of the ferromagnetic layer in a predetermined direction, wherein the antiferromagnetic layer is made of an antiferromagnetic material containing an element X and Mn, where the element X is from the group of elements consisting of Pt, Pd, Ir, Rh, Ru, and Os, and combinations thereof and combinations thereof, and wherein the crystalline structure of at least part of the antiferromagnetic layer has a CuAu0I type face-centered square ordered lattice.
The exchange film in accordance with the invention is a structure which is obtained through a heat treatment after deposition of the antiferromagnetic layer and the ferromagnetic layer.
One of the features of the present invention is that the antiferromagnetic layer has a region in which the ratio of the atomic percent of the element X to Mn increases towards the ferromagnetic layer. The presence of such a region indicates that the antiferromagnetic layer has been properly transformed from a disordered lattice to an ordered lattice without being restrained by factors, such as the crystalline structure of the ferromagnetic layer, at the interface between the antiferromagnetic layer and the ferromagnetic layer. Thus, the exchange coupling film in accordance with the present invention produces a greater exchange coupling magnetic field than those or the prior art.
The creation of this transformed region results from a production process which will be described later. Thus, in accordance with the present invention, the antiferromagnetic layer has a region in which the ratio of the atomic percent of the element X to Mn increases in a direction towards the ferromagnetic layer, and the crystalline structure of at least part of the antiferromagnetic layer has an ordered lattice. An important factor to these features is the structure of the antiferromagnetic layer as deposited, i.e., in the state prior to the heat treatment.
In accordance with the present invention, the antiferromagnetic layer is formed, for example, as follows. An ordered crystalline structure is readily formed and the antiferromagnetic properties are maximized when the ratio of Pt and Mn is set to 50:50. Such a composition ratio, however, serves to suppress the creation of a non-aligned crystal lattice state at the interface between ferromagnetic layer and the antiferromagnetic layer, resulting in an insufficient transformation from a disordered lattice to an ordered lattice under heat treatment and, hence, insufficient strength of the exchange coupling magnetic field.
Pt contents below about 50 at % tend to hamper transformation into an ordered lattice when heat-treated and, accordingly, make it difficult to achieve satisfactory antiferromagnetic properties. In addition, a strongly aligned crystal lattice state is developed at the interface between the antiferromagnetic layer and the ferromagnetic layer, so that the resultant exchange coupling magnetic field is unacceptably small. Conversely, Pt contents exceeding about 50 at % also hamper transformation to an ordered lattice when the antiferromagnetic layer is formed by heat treatment, so that only very small exchange coupling magnetic field is about 50 at % also hamper transformation to an ordered lattice when the antiferromagnetic layer is formed by heat treatment, so that only very small exchange coupling magnetic field is obtainable. Although such contents promote creation of non-aligned crystal lattice state at the interface between the antiferromagnetic layer and the ferromagnetic layer.
In accordance with the present invention, therefore, the antiferromagnetic layer is deposited to include a comparatively thin alloy film (referred to as xe2x80x9cfirst antiferromagnetic layerxe2x80x9d), which contacts the ferromagnetic layer and which is rich in Pt to facilitate creation of a non-aligned crystal lattice state at the interface adjacent to the ferromagnetic layer, and a Ptxe2x80x94Mn alloy film (referred to as xe2x80x9csecond antiferromagnetic layerxe2x80x9d) on the ferromagnetic layer through the intermediary of the first antiferromagnetic layer. The Ptxe2x80x94Mn alloy has a composition that permits easy transformation from a disordered lattice to an ordered lattice and has a thickness greater than that of the first antiferromagnetic layer.
Thus, in the Pt-enriched first antiferromagnetic layer at the interface adjacent to the ferromagnetic layer in the as-deposited structure prior to the heat treatment, the crystalline structure of the antiferromagnetic layer is not restrained by the structure of the ferromagnetic layer. Accordingly, the second antiferromagnetic layer, having a composition easy for transformation from a disordered lattice to an ordered lattice, is properly transformed as a result of the heat treatment. When the transformation is started by a heat treatment, diffusion of elements also takes place between the first and second antiferromagnetic layers. Accordingly, the Pt content in the region that has been constituted by the first antiferromagnetic layer is changed from that in the as-deposited state to create a composition which is rather easy to transform into an ordered lattice, with the result that the transformation takes place also in that region.
In the as-deposited state of the structure prior to heat treatment, the Pt-enriched first antiferromagnetic layer is interposed between the ferromagnetic layer and the second antiferromagnetic layer, where the second antiferromagnetic layer has an ideal composition that is easy to transform into CuAuxe2x80x94I face-centered square ordered lattice. Therefore, when a heat treatment is conducted, the crystalline structure of the antiferromagnetic layer can be sufficiently transformed from a disordered lattice to an ordered lattice, without being restrained by the factors such as crystalline structure of the ferromagnetic layer. Accordingly, an exchange coupling magnetic field greater than those of known devices can be obtained. Consequently, a composition modulation is effected by the heat treatment, so that the exchange coupling film, as heat-treated, has a region in which the ratio of the atomic percent of the element X increases in a direction towards the ferromagnetic layer. In addition, the crystalline structure of at least part of the antiferromagnetic layer has been changed to CuAuxe2x80x94I type face-centered square ordered lattice.
In accordance with the present invention, the antiferromagnetic layer may be made of an antiferromagnetic material containing an element X, an element Xxe2x80x2 and Mn, where the element X is selected from the group of elements consisting of Pt, Pd, Ir, Rh, Ru, and Os, and combinations thereof and the element Xxe2x80x2 is selected from the group of elements consisting of Ne, Ar, Kr, Xe, Be, B, C, N, Mg, Al, Si, P, Ti, V, Cr, Fe, 00, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, Cd, Sn, Hf, Ta, W, Re, Au, Pb and a rare earth element and combinations thereof. Also, the antiferromagnetic layer has a region in which the ratio of the atomic percent of the elements X+Xxe2x80x2 to Mn increases in a direction towards the ferromagnetic layer.
Preferably, the antiferromagnetic material containing an element X, an element Xxe2x80x2 and Mn is an interstitial solid solution in which the element Xxe2x80x2 has invaded and resides in the interstices of a space lattice constituted by the element X and Mn, or a substitutive solid solution in which part of the lattice points of a crystal lattice constituted by the element X and Mn has been substituted by the element Xxe2x80x2. The antiferromagnetic layer formed of an Xxe2x80x94Mnxe2x80x94Xxe2x80x2 alloy in the form of interstitial or substitutive solid solution can have a greater lattice constant than an Xxe2x80x94Mn alloy. This results in a greater difference in the lattice constant between the antiferromagnetic layer and the ferromagnetic layer, thus making it possible to maintain appropriate non-aligned crystal lattice state at the interface. The antiferromagnetic layer and the ferromagnetic layer may have different lattice constants or different crystalline orientations over at least part of the interface.
It is also preferred that a non-aligned crystal lattice state is created over at least part of the above-mentioned interface. The term xe2x80x9cnon-aligned crystal lattice statexe2x80x9d is used here to mean a state in which atoms in the antiferromagnetic layer and atoms in the ferromagnetic layer do not exactly correspond in a 1:1 fashion at the interface between the antiferromagnetic layer and the ferromagnetic layer.
Such a non-aligned crystal lattice state serves to reduce any restraint posed on the antiferromagnetic layer by the factors such as crystalline structure of the ferromagnetic layer. The non-aligned crystal lattice state contributes to realization of proper transformation from a disordered lattice to an ordered lattice, thereby making it possible to obtain a large exchange coupling magnetic field. In accordance with the present invention, the antiferromagnetic layer and the ferromagnetic layer have different lattice constants or different crystalline orientations over at least part of the interface therebetween. Such differences tend to cause the non-aligned crystal lattice state, thus offering a large exchange coupling magnetic field.
When the antiferromagnetic layer and the ferromagnetic layer have different crystalline orientations, assuming that the (111) planes of the ferromagnetic layers have been substantially oriented in parallel with the aforementioned interface, the degree of orientation of the (111) planes of the antiferromagnetic layer is smaller than that of the ferromagnetic layer. Alternatively, the (111) planes of the antiferromagnetic layer are not oriented at all. Likewise, assuming that the (111) planes of the antiferromagnetic layers have been substantially oriented in parallel with the aforementioned interface, the degree of orientation of the (111) planes of the ferromagnetic layer is smaller than that of the ferromagnetic layer, or, alternatively, the (111) planes are not oriented. Alternatively, the degrees of orientations of the (111) planes of both the antiferromagnetic layer and the ferromagnetic layer in directions parallel to the interface are small or these planes are not oriented, so that the crystal planes other than (111) planes are substantially oriented in parallel with the above-mentioned interface. Accordingly, a difference is created in the crystal orientation between the antiferromagnetic layer and the ferromagnetic layer.
The crystalline orientations are controllable by selectively using an underlying layer or adjusting values of factors such as composition ratios and electric power and gas pressure during the sputter deposition process, and suitably selecting the sequence of deposition of the films.
Preferably, assuming an imaginary boundary within the thickness of the antiferromagnetic layer in parallel with the interface so as to divide the antiferromagnetic layer into a first region between the imaginary boundary and the interface and a second region between the imaginary boundary and the face surface of the antiferromagnetic layer opposite to the interface, the antiferromagnetic has a region in which the ratio linearly or non-linearly increases from the second region towards the first region across the imaginary boundary. It is also preferred that the antiferromagnetic layer has a region in which the composition ratio (atomic percent) of the element X or the elements X+Xxe2x80x2 increases in a direction towards the ferromagnetic layer. It is also preferred that the antiferromagnetic layer has a region which is in proximity to the interface and in which the composition ratio (atomic percent) of the element X or the elements X+Xxe2x80x2 decreases in a direction towards the ferromagnetic layer.
More precisely, the above-described features mean that the region in which the atomic percent of the element X or the elements X+Xxe2x80x2 is maximized is not located near the aforementioned interface. Rather, there is a region in which the atomic percent of the element X or the elements X+Xxe2x80x2 is increased starting from the interface towards the side opposite to the interface. Thus, the atomic percent of the element X or the elements X+Xxe2x80x2 is maximized at a location which is spaced a predetermined distance in the thicknesswise direction from the interface.
The fact that the atomic percent of the element X or the elements X+Xxe2x80x2 is not maximized in a region near the interface is attributable to the fact that a diffusion of elements takes place between the antiferromagnetic layer (this is the xe2x80x9cfirst antiferromagnetic layerxe2x80x9d in the as-deposited state) and the ferromagnetic layer, as a result of the heat treatment which is conducted after the deposition. Thus, the atomic percent of the element X or the elements X+Xxe2x80x2 in the region near the interface is smaller in the as-heat-treated state than in the as-deposited state, so that the maximum atomic percent of the element X or the elements X+Xxe2x80x2 appears at a location spaced apart from the interface in the direction of thickness of the antiferromagnetic layer. The reduced atomic percent of the element X or the elements X+Xxe2x80x2 in the region near the interface permits proper transformation from a disordered lattice to an ordered lattice also in the region near the interface. Accordingly, a large exchange coupling magnetic field is obtained. Also, the antiferromagnetic layer may have a region that is near the face surface thereof opposite to the interface and in which the composition ratio (atomic percent) of the element X or the elements X+Xxe2x80x2 decreases in the direction away from the ferromagnetic layer.
Preferably, the composition ratio of the element X or the elements X+Xxe2x80x2 of the antiferromagnetic layer to the total composition ratio 100 at % of all the elements constituting the antiferromagnetic layer is about 50 at % to about 65 at % and, more preferably, about 50 at % to about 60 at %, in the region near the interface between the antiferromagnetic layer and the ferromagnetic layer.
As will be explained later, in the as-deposited state prior to the heat treatment, an antiferromagnetic layer serving as the first antiferromagnetic layer having a composition ratio of the element X of, for example, about 53 at % to about 65 at % is deposited at the interface adjacent to the ferromagnetic layer. The second antiferromagnetic layer formed on the first antiferromagnetic layer is deposited to have a composition in which the composition ratio of the element X is about 44 at % to about 57 at %.
When the first and second antiferromagnetic layers have composition ratios as specified above are deposited, a diffusion of elements takes place between the first antiferromagnetic layer and the second antiferromagnetic layer. Thus, the composition ratio of the element X may have fallen below 53 at % in the region near the interface after the heat treatment. In accordance with the present invention, therefore, the composition ratio of the element X or the elements X+Xxe2x80x2 in the state after the heat treatment is preferably about 50 at % to about 65 at %, in the region near the interface.
In accordance with the present invention, it is preferred also that the composition ratio of the element X of the antiferromagnetic layer to the total composition ratio 100 at % of all the elements constituting the antiferromagnetic layer is about 44 at % to about 57 at % and, preferably, about 46 at % but not greater than 55 at %, in the region near the face of the antiferromagnetic layer opposite to the interface adjacent to the ferromagnetic layer.
Preferably, the antiferromagnetic layer has a thickness of at least about 73 xc3x85. According to the present invention, it is possible to obtain a large exchange coupling magnetic field even with the antiferromagnetic layer having such a small thickness.
In accordance with the present invention, there is provided also an exchange coupling film comprising: an antiferromagnetic layer; and a ferromagnetic layer formed such that an exchange coupling magnetic field is produced at the interface between the antiferromagnetic layer and the ferromagnetic layer, wherein the antiferromagnetic layer is made of an antiferromagnetic material containing an element X and Mn, where the element X is selected from the group of elements consisting of Pt, Pd, Ir, Rh, Ru, and Os, and combinations thereof; wherein the antiferromagnetic layer has a region in which the ratio of the atomic percent of the element X or the elements X+Xxe2x80x2 to Mn increases in a direction towards the ferromagnetic layer starting from a thicknesswise central region, and a region in which the ratio of the atomic percent of the element X or the elements X+Xxe2x80x2 to Mn increases in the direction away from the ferromagnetic layer starting from the thicknesswise central region; and wherein the crystalline structure of at least part of the antiferromagnetic layer has a CuAuxe2x80x94I type face-centered square ordered lattice. The exchange coupling film as set forth above is a structure obtained through a heat treatment which is executed after the deposition of the layers.
In accordance with the present invention, the antiferromagnetic layer has a region in which the ratio of the atomic percent of the element X or the elements X+Xxe2x80x2 to Mn increases in a direction towards the ferromagnetic layer starting from a thicknesswise central region, and a region in which the ratio of the atomic percent of the element X or the elements X+Xxe2x80x2 to Mn increases in the direction away from the ferromagnetic layer starting from the thicknesswise central region. In addition, the crystalline structure of at least a portion of the antiferromagnetic layer has a CuAuxe2x80x94I type face-centered square ordered lattice.
The presence of these regions where a change or modulation of composition occurs is derived from the production process that will be described later. In the present invention, by virtue of the presence of these regions, the transformation takes place properly in the antiferromagnetic layer over its entirety, without being restrained by the crystalline structure of the ferromagnetic layer at the interface adjacent to the ferromagnetic layer and without being restrained by the crystalline structure of the layer contacting the antiferromagnetic layer at the opposite interface. Accordingly, the crystalline structure of part of the antiferromagnetic layer is changed into CuAuxe2x80x94I face-centered square ordered lattice structure,
Thus, in accordance with the present invention, the antiferromagnetic layer after the heat treatment has a region in which the ratio of the atomic percent of the element X to Mn increases in a direction towards the ferromagnetic layer, and a region in which the above-mentioned ratio increases in the direction away from the ferromagnetic layer, and the crystalline structure of at least part of the antiferromagnetic layer has ordered lattice. An important factor to these features is the structure of the antiferromagnetic layer as deposited, i.e., in the state prior to the heat treatment.
In accordance with the present invention, the antiferromagnetic layer is formed, for example, as follows. An ordered crystalline structure is easily formed and the antiferromagnetic properties are maximized when the ratio of content between Pt and Mn is set to 50:50. Such a composition ratio, however, serves to suppress creation of non-aligned crystal lattice state at the interface adjacent to the ferromagnetic layer, resulting in an insufficient transformation from a disordered lattice to an ordered lattice under heat treatment and, hence, insufficient strength of the exchange coupling magnetic field. Pt contents below 50 at % tend to hamper transformation into ordered lattice when heat-treated and, accordingly, make it difficult to achieve satisfactory antiferromagnetic properties. In addition, a strongly aligned state is developed at the interface between the antiferromagnetic layer and the ferromagnetic layer, so that the resultant exchange coupling magnetic field is unacceptably small. Conversely, Pt contents exceeding 50 at % also hampers transformation to an ordered lattice when the antiferromagnetic layer is formed by heat treatment, so that only a very small exchange coupling magnetic field is obtainable even though such contents promote creation of non-aligned crystal lattice state at the interface between the antiferromagnetic layer and the ferromagnetic layer.
In accordance with the present invention, therefore, the antiferromagnetic layer is deposited to include a comparatively thin alloy film (referred to as xe2x80x9cfirst antiferromagnetic layerxe2x80x9d) which contacts the ferromagnetic layer and which is rich in Pt to facilitate creation of a non-aligned crystal lattice state at the interface adjacent to the ferromagnetic layer. A Ptxe2x80x94Mn alloy film (referred to as xe2x80x9csecond antiferromagnetic layerxe2x80x9d) is formed on the first antiferromagnetic layer where the Ptxe2x80x94Mn alloy has a composition that permits easy transformation from a disordered lattice to an ordered lattice and has a thickness greater than that of the first antiferromagnetic layer. In addition, a Pt-enriched third antiferromagnetic layer is formed in contact with the second antiferromagnetic layer.
Thus, in the Pt-enriched first antiferromagnetic layer at the interface adjacent to the ferromagnetic layer in the as-deposited structure prior to the heat treatment, the crystalline structure of the antiferromagnetic layer is not restrained by the structure of the ferromagnetic layer. Accordingly, the second antiferromagnetic layer, having a composition easy for transformation from a disordered lattice to an ordered lattice, is properly transformed as a result of the heat treatment. When the transformation is started by a heat treatment, diffusion of elements also takes place between the first and second antiferromagnetic layers, as well as between the third antiferromagnetic layer and the second antiferromagnetic layer. Accordingly, the Pt content in the region that has been constituted by the first antiferromagnetic layer, as well as in the region that has been constituted by the third antiferromagnetic layer, is changed from that in the as-deposited state to create a composition which is rather easy to transform into an ordered lattice, with the result that the transformation takes place also in the regions that have been constituted by the first and third antiferromagnetic layers in the as-deposited state.
In the as-deposited state of the structure prior to heat treatment, the Pt-enriched first and third antiferromagnetic layers are formed to sandwich therebetween the second antiferromagnetic layer having an ideal composition that is easy to be transformed into CuAuxe2x80x94I face-centered square ordered lattice. Therefore, when a heat treatment is conducted, the crystalline structure of the antiferromagnetic layer can be sufficiently transformed from a disordered lattice to an ordered lattice. Accordingly an exchange coupling magnetic field greater than those of known devices can be obtained.
In the heat-treated exchange coupling film formed as described above, a composition modulation has occurred, so that a region exists in which the ratio of the atomic percent of the element X increases in a direction towards the ferromagnetic layer starting from a thicknesswise central region, and a region in which the ratio increases in a direction towards the side opposite to the ferromagnetic layer. In addition, the crystalline structure of at least part of the antiferromagnetic layer has a CuAuxe2x80x94I type face-centered square ordered lattice.
The Pt-enriched third antiferromagnetic layer, which is deposited in contact with the second antiferromagnetic layer, serves to promote proper ordered transformation also in the region of the antiferromagnetic layer opposite to the ferromagnetic layer, thus ensuring a proper transformation over the entirety of the antiferromagnetic layer. Accordingly, a large exchange coupling magnetic field can be obtained.
The layer deposited to the side of the antiferromagnetic layer opposite to the ferromagnetic layer may be a non-magnetic underlying layer, a protective layer, or a seed layer. Factors such as the crystalline structure of such a layer may impede an ordered transformation of the antiferromagnetic layer, leading to a reduction in the exchange coupling magnetic field. In this invention, the Pt-enriched third antiferromagnetic layer is deposited on the above-mentioned side of the antiferromagnetic layer, thereby promoting ordered transformation of the antiferromagnetic layer, while avoiding the influence of the above-mentioned crystalline structure. Accordingly, a large exchange coupling magnetic field is obtained.
In accordance with the present invention, there is provided also an exchange coupling film comprising: an antiferromagnetic layer; and a ferromagnetic layer formed such that an exchange coupling magnetic field is produced at the interface between the antiferromagnetic layer and the ferromagnetic layer, wherein the antiferromagnetic layer is made of an antiferromagnetic material containing an element X, an element Xxe2x80x2 and Mn, where the element X is selected from the group of elements consisting of Pt, Pd, Ir, Rh, Ru, and Os, and combinations thereof, and the element Xxe2x80x2 is selected from the group of elements consisting of Ne, Ar, Kr, Xe, Be, B, C, N, Mg, Al, Si, P, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, Cd, Sn, Hf, Ta, W, Re, Au, Pb and a rare earth element and combinations thereof; wherein the antiferromagnetic layer has a region in which the ratio of the atomic percent of the elements X+Xxe2x80x2 to Mn increases in a direction towards the ferromagnetic layer starting from a thicknesswise central region and a region in which the ratio of the atomic percent of the elements X+Xxe2x80x2 to Mn increases in the direction away from the ferromagnetic layer starting from the thicknesswise central region; and wherein the crystalline structure of at least a portion of the antiferromagnetic layer has a CuAuxe2x80x94I type face-centered square ordered lattice.
In this aspect of the invention, the antiferromagnetic layer is made from an antiferromagnetic material containing an element X, an element Xxe2x80x2 and Mn. In this aspect also, the antiferromagnetic layer is formed of the above-described three layers, and the composition ratio of the element X+Xxe2x80x2 in the second antiferromagnetic layer is set to be smaller than those in the first and third antiferromagnetic layers. A heat treatment after the deposition triggers transformation in the second antiferromagnetic layer and transformation takes place also in the first and third antiferromagnetic layer. Consequently, transformation to ordered lattice structure occurs over the entirety of the antiferromagnetic layer. Accordingly, an exchange coupling magnetic field greater than those obtainable with known devices can be obtained.
In the state after the heat treatment, the antiferromagnetic layer has a region in which the ratio of the atomic percent of the elements X+Xxe2x80x2 to Mn increases in a direction towards the ferromagnetic layer starting from the thicknesswise central region, and a region in which the above-mentioned ratio increases in the direction away from the ferromagnetic layer. In addition, the crystalline structure of at least part of the antiferromagnetic layer has CuAuxe2x80x94I type face-centered square ordered lattice.
Preferably, the alloy material containing an element X, an element Xxe2x80x2 and Mn is an interstitial solid solution in which the element Xxe2x80x2 has invaded and resides in the interstices of a space lattice constituted by the element X and Mn, or a substitutive solid solution in which part of the lattice points of a crystal lattice constituted by the element X and Mn are substituted by the element Xxe2x80x2.
It is also preferred that the above-described exchange coupling lattice further comprises a seed layer formed on the side of the antiferromagnetic layer opposite to the interface adjacent to the ferromagnetic layer, the seed layer having face-centered cubic crystals with substantially oriented (111) planes, the antiferromagnetic layer and the ferromagnetic layer having crystalline orientations with the (111) planes substantially oriented in parallel with the interface.
The seed layer serves to promote the substantial orientation of the crystalline structures of the antiferromagnetic layer and the ferromagnetic layer, thus allowing the crystal grains to grow large, thereby offering a large ratio of resistance variation. Preferably, the antiferromagnetic layer and the seed layer have different lattice constants over at least part of the interface therebetween.
It is also preferred that a non-aligned crystal lattice state is created over at least part of the interface between the antiferromagnetic layer and the seed layer. Such features are attained by, for example, a process in which the antiferromagnetic layer is formed by depositing three films, such that the Pt-enriched third antiferromagnetic layer is formed in the portion to be contacted by the seed layer. In the as-deposited state, a non-aligned crystal lattice state exists over at least part of the interface between the third antiferromagnetic layer and the seed layer. Therefore, when the heat treatment is conducted, transformation into ordered lattice structure properly takes place in the antiferromagnetic layer, while the non-aligned crystal lattice state is maintained at the interface between the antiferromagnetic layer and the seed layer. Accordingly, an exchange coupling magnetic field greater than those obtainable with known devices can be achieved.
Preferably, the seed layer is formed of an Nixe2x80x94Fe alloy or an Nixe2x80x94Fexe2x80x94Y alloy, wherein Y is an element selected from the group consisting of Cr, Rh, Ta, Hf, Nb, Zr, and Ti and combinations thereof.
It is also preferred that the seed layer is a non-magnetic layer. It is important that the seed layer has a large specific resistance. High specific resistance of the seed layer suppresses shunting of the sense current into the seed layer, thereby offering advantages such as increase in the resistance variation ratio and reduction in the Barkhausen noise.
Preferably, the exchange coupling film is formed by sequentially depositing, on an underlying layer, the seed layer, the antiferromagnetic layer, and the ferromagnetic layer. The underlying layer is formed of at least one element selected from the group consisting of Ta, Hf, Nb, Zr, Ti, Mo and W.
A layer formed of elements selected from the group consisting of Ta, Hf, Nb, Zr, Ti, Mo and W and combinations thereof may be formed on the side of the antiferromagnetic layer opposite to the interface adjacent to the ferromagnetic layer.
In accordance with the invention, it is preferred that, assuming a first imaginary boundary at the side of the thicknesswise center of the antiferromagnetic layer adjacent to the face surface of the antiferromagnetic layer opposite to the interface and a second imaginary boundary at the side of the thicknesswise center adjacent to the interface, the ratio is greater in a first region between the face surface of the antiferromagnetic layer and the first imaginary boundary and in a third region between the interface adjacent to the ferromagnetic layer and the second imaginary boundary than in a second region between the first and second imaginary boundaries. Also, the ratio linearly or non-linearly increases from the second region towards the first region across the first imaginary boundary and from the second region towards the third region across the second imaginary boundary.
The antiferromagnetic layer preferably includes a region in which the atomic percent of the element X increases in a direction towards the interface adjacent to the ferromagnetic layer starting from a predetermined thicknesswise central region, and a region in which the atomic percent of the element X increases in a direction towards the side opposite to the interface starting from the predetermined thicknesswise central region.
It is also preferred that the antiferromagnetic layer includes a region in which the atomic percent of the element X decreases in a direction towards the interface adjacent to the ferromagnetic layer, and a region in which the atomic percent of the element X decreases in a direction towards the side opposite to the interface.
More precisely, the above-described features mean that the region in which the atomic percent of the element X or the elements X+Xxe2x80x2 is maximized is not located near the aforementioned interface. Rather, there is a region in which the atomic percent of the element X or the elements X+Xxe2x80x2 is increased starting from the interface towards the side opposite to the interface. Thus, the atomic percent of the element X or the elements X+Xxe2x80x2 is maximized at a location which is spaced a predetermined distance in the thicknesswise direction from the interface.
The fact that the atomic percent of the element X or the elements X+Xxe2x80x2 is not maximized in a region near the interface is attributable to the fact that a diffusion of elements takes place at the interface between the antiferromagnetic layer and the ferromagnetic layer, as well as at the interface between the antiferromagnetic layer and the layer formed on the side opposite to the ferromagnetic layer, as a result of the heat treatment which is conducted after the deposition. Thus, the atomic percent of the element X or the elements X+Xxe2x80x2 in the region near the interface is smaller in the as-heat-treated state than in the as-deposited state, so that the maximum atomic percent of the element X or the elements X+Xxe2x80x2 appears at a location spaced apart from the interface in the direction of thickness of the antiferromagnetic layer. The reduced atomic percent of the element X or the elements X+Xxe2x80x2 in the region near the interface permits proper transformation from a disordered lattice to an ordered lattice also in the region near the interface. Accordingly, a large exchange coupling magnetic field is obtained.
Preferably, the composition ratio of the element X or the elements X+Xxe2x80x2 of the antiferromagnetic layer to the total composition ratio 100 at % of all the elements constituting each of the portions of the antiferromagnetic layer near the interface adjacent to the ferromagnetic layer and near the side opposite to the interface is about 50 at % to about 65 at %, in the region near the interface between the antiferromagnetic layer and the ferromagnetic layer, as well as at the side opposite to the above-mentioned interface.
As will be explained later, in the as-deposited state prior to the heat treatment, an antiferromagnetic layer serving as the first antiferromagnetic layer having a composition ratio of the element X or the elements X+Xxe2x80x2 of, for example, about 53 at % to about 65 at % is deposited at the interface adjacent to the ferromagnetic layer, and an antiferromagnetic layer serving as the third antiferromagnetic layer and having a composition ratio of the element X or the elements X+Xxe2x80x2 of, for example, about 53 at % to about 65 at % is deposited on the first antiferromagnetic layer through the intermediary of a second antiferromagnetic layer. The second antiferromagnetic layer formed between the first antiferromagnetic layer and the third antiferromagnetic layer has a composition in which the composition ratio of the element X or the elements X+Xxe2x80x2 is about 44 at % to about 57 at %.
When the first, second and third antiferromagnetic layers having composition ratios as specified above are deposited, a diffusion of elements takes place between the first antiferromagnetic layer and the ferromagnetic layer, as well as between the third antiferromagnetic layer and the layer formed on the side of the antiferromagnetic layer opposite to the ferromagnetic layer. Thus, the composition ratio of the element X or the elements X+Xxe2x80x2 may have fallen below 53 at % in the region near each interface, in the state after the heat treatment. In accordance with the present invention, therefore, the composition ratio of the element X or the elements X+Xxe2x80x2 in the state after the heat treatment is preferably about 50 at % to about 65 at %, in the region near the interface.
Preferably, the antiferromagnetic layer has a thickness of at least about 76 xc3x85. In accordance with the present invention, it is possible to achieve a large exchange coupling film, even with the antiferromagnetic layer having such a small thickness.
The antiferromagnetic layer and the ferromagnetic layer may have different lattice constants, or different crystalline orientations, over at least part of the interface therebetween. In this invention, the antiferromagnetic layer and the ferromagnetic layer have the same crystalline orientation when the seed layer is used. Preferably, a non-aligned crystal lattice state exists over at least part of the above-mentioned interface. The presence of such a non-aligned crystal lattice state promotes the transformation of the antiferromagnetic layer into an ordered lattice, thus making it possible to obtain a large exchange coupling magnetic field.
When the antiferromagnetic layer and the ferromagnetic layer have different crystalline orientations, assuming that the (111) planes of the ferromagnetic layers have been substantially oriented in parallel with the aforementioned interface, the degree of orientation of the (111) planes of the antiferromagnetic layer is smaller than that of the ferromagnetic layer. Alternatively, the (111) planes of the antiferromagnetic layer are not oriented. Likewise, assuming that the (111) planes of the antiferromagnetic layers have been substantially oriented in parallel with the aforementioned interface, the degree of orientation of the (111) planes of the ferromagnetic layer is smaller than that of the ferromagnetic layer, or the (111) planes are not oriented.
Alternatively, the degrees of orientations of the (111) planes of both the antiferromagnetic layer and the ferromagnetic layer in directions parallel to the interface are small or these planes are not oriented, so that the crystal planes other than (111) planes are substantially oriented in parallel with the above-mentioned interface. Accordingly, a difference is created in the crystal orientation between the antiferromagnetic layer and the ferromagnetic layer. The crystalline orientations are controllable by selectively using an underlying layer or adjusting values of factors such as composition ratios and electric power and gas pressure during the sputtering, and suitably selecting the sequence of deposition of the films.
The exchange coupling film of the present invention having the above-described features is usable in a variety of types of magnetoresistive sensor. For example, the present invention provides a magnetoresistive sensor comprising: an antiferromagnetic layer; a pinned magnetic layer in contact with the antiferromagnetic layer such that an exchange coupling magnetic field is produced at the interface between the antiferromagnetic layer and the pinned magnetic layer to fix the magnetization of the pinned magnetic layer in a predetermined direction; a non-magnetic intermediate layer between the pinned magnetic layer and a free magnetic layer; and a bias layer which aligns the direction of magnetization of the free magnetic layer in a direction intersecting the direction of magnetization of the pinned magnetic layer; wherein the antiferromagnetic layer and the pinned magnetic layer comprise the exchange coupling film described above.
The present invention also provides a magnetoresistive sensor comprising: an antiferromagnetic layer; and a pinned magnetic layer in contact with the antiferromagnetic layer such that an exchange coupling magnetic field is produced at the interface between the antiferromagnetic layer and the pinned magnetic layer to fix the magnetization of the pinned magnetic layer in a predetermined direction; a non-magnetic intermediate layer between the pinned magnetic layer and a free magnetic layer; and an antiferromagnetic exchange bias layer adjacent to one of an upper side or a lower side of the free magnetic layer and having portions spaced from each other in a track width direction; wherein the exchange bias layer and the free magnetic layer comprise the exchange coupling film described above; and wherein the magnetization of the free magnetic layer is fixed in a predetermined direction.
The present invention also provides a magnetoresistive sensor comprising: a free magnetic layer; first and second non-magnetic intermediate layers formed on upper and lower sides of the free magnetic layer, respectively; first and second pinned magnetic layers, wherein the first pinned magnetic layer is formed on an upper side of the first non-magnetic intermediate layers and the second pinned magnetic layer on a lower side of the second non-magnetic intermediate layer; first and second antiferromagnetic layers, wherein the first ferromagnetic layer is formed on an upper side of the first pinned magnetic layers and the second ferromagnetic layer on a lower side of the second pinned magnetic layer, the first and second antiferromagnetic layers serving to fix the directions of magnetization of the first and second pinned magnetic layers, respectively, by exchange anisotropic magnetic fields; and a bias layer adjacent to sides of the free magnetic layer which aligns the direction of magnetization of the free magnetic layer to a direction that intersects the directions of the pinned magnetic layers; wherein at least one of the first antiferromagnetic layer and the first pinned magnetic layer or the second antiferromagnetic layer and second pinned magnetic layer are formed of the exchange coupling film described above.
The present invention also provides a magnetoresistive sensor comprising: a non-magnetic layer; a magnetoresistive layer and a soft magnetic layer separated by the non-magnetic layer; and an antiferromagnetic layer formed on one of an upper side or a lower side of the magnetoresistive layer and having portions spaced from each other in a track width direction; wherein the antiferromagnetic layer and the magnetoresistive layer and the magnetoresistive layer comprise the exchange coupling film described above.
The present invention also provides a thin-film magnetic head having shield layers formed on gap layers adjacent to the upper and lower sides of the magnetoresistive sensor.